1. Field of the Invention
The present invention relates to a lens drive system applied in an autofocus operation, and more particularly, to a lens drive system of a camera that uses a contrast-detection autofocus process.
2. Description of the Related Art
In the contrast-detection autofocus operation, a lens position where the contrast of an image appears to be the maximum value (a peak position) is discovered by translating the lens. In order to detect a peak position, it is necessary for the lens to pass once over the peak position, which corresponds to the in-focus position. Namely, when the peak position is detected, it is necessary to move the lens back to the peak position to adjust the lens system to the in-focus condition. However, every lens drive system includes backlash and the exact size of the backlash is normally unknown. Therefore, when the peak position is detected, the lens is moved back a distance that is greater than a distance sufficient to counterbalance the exceeded travel and the backlash. In turn, the lens is again translated forward, as in the searching operation, until an image having the contrast equal to the above-detected peak value is obtained. Thereby, the lens system is positioned in an in-focus position.
In consideration of the variation of backlash among lenses, camera bodies, and lens types, Japanese Unexamined Patent Publication No. H05-297437, which relates to a phase-difference detection method, discloses a camera system where backlash data is stored in a memory inside a lens barrel to calibrate the lens drive and to achieve a precise distance measurement.